Gas-operated reloading

Gas-operated firearm (long-stroke piston, e.g. AK-47). 1) gas port, 2) piston head, 3) rod, 4) bolt, 5) bolt carrier, 6) spring

Gas-operation is a system of operation used to provide energy to operate autoloading firearms. In gas operation, a portion of high-pressure gas from the cartridge being fired is used to power a mechanism to extract the spent case and chamber a new cartridge. Energy from the gas is harnessed through either a port in the barrel or a trap at the muzzle. This high-pressure gas impinges on a surface such as a piston head to provide motion for unlocking of the action, extraction of the spent case, ejection, cocking of the hammer or striker, chambering of a fresh cartridge, and locking of the action. The first gas-operated rifle was designed by Karel Krnka in 1883.

Gas systems

Most current gas systems employ some type of piston. The face of the piston is acted upon by gas from the combustion of the propellant from the barrel of the firearm. Early methods such as Browning's 'flapper' prototype, the Bang rifle, and Garand rifle used relatively low-pressure gas from at or near the muzzle. This, combined with more massive operating parts, reduced the strain on the mechanism. To simplify and lighten the firearm, gas from nearer the chamber needed to be used. This gas is of extremely high pressure and has sufficient force to destroy a firearm unless it is regulated somehow. Several methods are employed to regulate the energy. The M1 carbine incorporates a very short piston, or "tappet". This movement is closely restricted by a shoulder recess. Excess gas is then vented back into the bore. The M14 rifle and M60 GPMG use the White expansion and cutoff system to stop (cut off) gas from entering the cylinder once the piston has traveled a short distance.[1] Most systems, however, vent excess gas into the atmosphere through slots, holes, or ports.

Long-stroke piston

Diagram of long-stroke gas operation system
Long stroke gas piston, from an AK-74.

With a long-stroke system, the piston is mechanically fixed to the bolt group and moves through the entire operating cycle. This system is used in weapons such as the Bren light machine gun, AK-47, Tavor, FN Minimi, M249 Squad Automatic Weapon, FN MAG, FN FNC, and M1 Garand. The primary advantage of the long-stroke system, beyond design simplicity and robustness, is that the mass of the piston rod adds to the momentum of the bolt carrier enabling more positive extraction, ejection, chambering, and locking. Also, as the gas is not directed back into the chamber, the weapon stays cleaner longer thus reducing the likelihood of a malfunction. The primary disadvantage to this system is the disruption of the point of aim due to the center of mass changing during the action cycle, and energetic and abrupt stops at the beginning and end of bolt carrier travel. Also, due to the greater mass of moving parts, more gas is required to operate the system.

Direct impingement

direct impingement
Main article: Direct impingement

The direct impingement (DI) method of operation vents gas through a tube to the working parts of a rifle where they directly impinge on the bolt carrier. Firearms that use this system include the French MAS-40 from 1940, the Swedish Ljungman Ag m/42 from 1942, and the American M16 rifle from 1963 and its many succeeding copies and derivatives. This system has the advantage of having the absolute minimum of recoiling action parts, resulting in the minimum possible weapon disturbance due to balance shifting during the action cycle as well as reducing overall weapon weight. It has the disadvantage of the propellant gas (and the accompanying fouling) being blown directly into the action parts.[2] DI operation increases the amount of heat that is deposited in the receiver while firing, which can burn off essential lubricants. Lack of proper lubrication is the most common source of weapon stoppages or jams. The bolt, extractor, ejector, pins, and springs are also heated by this high-temperature gas. These combined factors reduce service life of these parts, reliability, and mean time between failures.[3]

Short-stroke piston

short-stroke gas piston
Short stroke gas piston and bolt carrier group, from a gas piston AR-15.

With a short-stroke or tappet system, the piston moves separately from the bolt group. It may directly push[4] the bolt group parts as in the M1 carbine or operate through a connecting rod or assembly as in the Armalite AR-18 or the SKS. In either case, the energy is imparted in a short, violent push and the motion of the gas piston is then arrested allowing the bolt carrier assembly to continue through the operating cycle through kinetic energy. This has the advantage of reducing the total mass of recoiling parts compared to the long-stroke piston. This, in turn, enables better control of the weapon due to less mass needing to be stopped at either end of the bolt carrier travel.

Gas trap

A gas trap system is similar to long-stroke operation; however, gas is 'trapped' after leaving the muzzle. Often called the 'Bang principle' for the Danish Bang rifle, other rifles to use this method were early production US 'gas-trap' M1 Garands, German Gewehr 41 (both Walther and Mauser models), and Chinese General Liu rifle. John Browning was the first to use gas trapped at the muzzle to operate a 'flapper' that, in turn, operated the mechanism in one of the earliest designs for a gas-operated firearm. These designs would eventually mature into the Colt M1895 machine gun. These systems are longer, heavier, dirtier and more complex; however, they used lower pressure gas and did not require a hole in the barrel, two advantages that are largely negated by their disadvantages.

Gas assist systems

Floating chamber

Early machine guns were expensive to operate. The United States Army wanted to train machine gun crews with less-expensive ammunition. To do this, they needed the .22 LR cartridge to operate firearms designed to use the .30-06 cartridge. David Marshall Williams invented a method that involved a separate floating chamber that acted as a gas piston with combustion gas impinging directly on the front of the floating chamber.[5] The .22 caliber Colt Service Ace conversion kit for the .45 caliber M1911 pistol also used Williams' system, which allows a much heavier slide than other conversions operating on the unaugmented blowback mechanism and makes training with the converted pistol realistic. A floating chamber provides additional force to operate the heavier slide, providing a felt recoil level similar to that of a full power cartridge.[6]

Muzzle booster

Animation of the Vickers muzzle booster operation, showing the expanding gases pushing the barrel to the rear relative to the cooling jacket
Main article: Muzzle booster

The French Chauchat, German MG-42 machine gun, the British Vickers machine gun, and some other recoil operated firearms use a gas trap style mechanism to provide additional energy to 'boost' the energy provided by recoil. This 'boost' provides higher rates of fire and/or more reliable operation. It is alternately called a gas assist, and may also be found in some types of blank-firing adapters.

Gas-delayed blowback

The bolt is not locked but is pushed rearward by the expanding propellant gases as in other blowback-based designs. However, propellant gases are vented from the barrel into a cylinder with a piston that delays the opening of the bolt. It is used by Volkssturmgewehr 1-5 rifle, the Heckler & Koch P7 and Steyr GB pistols.

Gas ejection

Patented by August Schuler, the Reform pistol featured a vertical row of barrels that advanced upwards with each shot exposing the fired chamber. As the lower barrel fired, a gas hole between the barrels pressurized the empty barrel enough to eject the case rearward. An extended spur on the hammer prevented the spent case from hitting the firer in the face. The final case required manual extraction.

See also


  1. U.S. Patent 1,907,163
  2. Smith, W.H.B.; Ezell, E. C. (1983), Small Arms of the World, 12th Edition, Stackpole Company, Harrisburg PA
  3. Major Thomas P. Ehrhart Increasing Small Arms Lethality in Afghanistan: Taking Back the Infantry Half-Kilometer. US Army. 2009
  4. U.S. Patent 2,090,656 Page 8, column 2, lines 67-70, Pg 9, column 1, lines 22-39
  5. Charles E. Petty, "Delightful diversion: testing Kimber's new rimfire was a tough job, but someone had to do it", Guns Magazine, March 2004. Contains some discussion about the floating chamber device.
  6. S. P. Fjestad (1991). Blue Book of Gun Values (13th ed.). p. 291. ISBN 0-9625943-4-2.
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